The Open Systems Interconnection (OSI) model is a model that standardizes the internal functions of a communication system by partitioning it into abstraction layers. The OSI model groups similar communication functions into one of seven different logical layers. Each layer, except the top layer, serves the layer above it, and each layer, except the bottom layer, is served by the layer below it. The seven layers of the OSI model include, from top layer #7 to bottom layer #1, the application layer, the presentation layer, the session layer, the transport layer, the network layer, the data link layer, and the physical layer. The application, presentation, session and transport layers comprise the host layers, and the network, data link and physical layers comprise the media layers. The application, presentation and session layers handle/process service data units (SDUs) that comprise merely data. The transport layer handles/processes SDUs that comprise segments. The network layer handles/processes SDUs that comprise packets/datagrams. The data link layer handles/processes SDUs that comprise bits and frames. The physical layer handles/processes SDUs that comprise bits. A SDU is a specific unit of data that has been passed down from a higher OSI layer to a lower OSI layer, and which the lower layer has not yet encapsulated into a protocol data unit (PDU). The SDU at a layer N, is the PDU of layer N+1. The OSI layer N−1 adds headers and/or footers, to the SDU to transform it into a PDU of layer N−1. The added headers and/or footers make it possible to get the data from a source network node to a destination network node.
The application layer interacts directly with the software application and typically involves functions such as identifying communication partners, determining resource availability, and synchronizing communication. In Transmission Control Protocol (TCP)/Internet Protocol (IP), application layer implementations may include Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), and Simple Network Management Protocol (SNMP). The presentation layer establishes context between application-layer entities and translates between application and network formats. The presentation layer formats and encrypts data to be sent across a network.
The session layer controls the connections between endpoint network nodes. The session layer establishes, manages and terminates the connections between the local and remote application. The transport layer provides the reliable communication of data between nodes located on a network. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. The network layer transfers variable length data sequences (i.e., datagrams) from one node to a next node in a network. The network layer provides mechanisms for routing messages in the network. The data link layer provides a reliable link between two directly connected nodes by detecting and correcting errors that occur in the physical layer. The physical layer performs a number of functions, including defining the electrical and physical specification of the data connection, defining the protocol to establish and terminate a connection, defining the protocol for flow control, and defining a protocol for the provision of a connection between two directly connected nodes.